The invention relates to fuses, and particularly relates to fuse indicators for indicating the status of a fuse.
Although fuses are ubiquitous in electrical systems, serving to protect the electrical systems themselves as well as the safety of persons using the systems, the detection of whether a fuse has been overcharged, (or blown), is often expensive and time consuming, particularly if an electrical system includes numerous fuses. Many fuses undergo no physical change in appearance upon being overcharged. Moreover, sometimes the contacts within a fuse may separate due to the fuse having undergone too many cycles of on-off use or too many cycles between widely varying temperatures, leaving no visible indication of having developed an open circuit. Fuse indicators have been developed to permit more rapid identification of the status of a fuse, typically by visual inspection of an indicator portion of a fuse.
Conventional fuse indicators generally include either a current sensor circuit that provides a visible indication of whether current is flowing through the fuse, or include a second fusable conductor path in parallel with the fuse filament. Fuse indicators with current sensor circuits include those disclosed in U.S. Pat. No. 4,641,120 which discloses a current sensor circuit that uses the current flowing through the fuse to light a light emitting diode (LED), and U.S. Pat. No. 2,164,658 which discloses a current sensor circuit including a lamp and resistor in parallel with the fuse. Such fuse indicators, however, are typically expensive to manufacture and must be carefully handled so as to not disturb the current sensor circuit. Further, such fuse indicators require that the power to a circuit be on in order to identify whether the LED or lamp should be activated. This may be not only inconvenient, but dangerous as well.
Fuse indicators that include a second conductor path are typically designed such that the second conductor path has a higher resistance than the fusable filament, and the second conductor path undergoes a visible change when subjected to excess current. During use, when excess current flows through the fuse filament (i.e., when the fuse is activated by overheating, or blows), then the current will travel the second conductor path and immediately cause it to undergo the desired visible change as the second conductor path is overheated, leaving an open circuit. For example, U.S. Pat. No. 1,793,103 discloses a fuse indicator, including a fusible wire within a transparent glass tube. Such fuse indicators, however, are also generally expensive and require delicate handling to ensure that the second conductor path is not disturbed.
Although the above types of fuse indicators have been known for quite some time, the need remains for a fuse indicator that reliably indicates the status of a fuse, yet is inexpensive to manufacture, is easily handled, and is convenient to use.